This invention relates to a circuit for producing an extremely linear frequency modulated audio carrier for use in a video signal distribution system, such as a cable television system.
The headend of a cable television distribution system frequency modulates the program audio onto a carrier which is offset 4.5 MHz from the program video carrier. In the past, headend distribution systems have utilized vaiable oscillators for generating the frequency modulated audio carrier. The variable oscillators have generally been voltage controlled oscillators operated at RF frequencies (i.e. above 1 MHz). The linearity of the audio carrier produced by such circuitry, although generally suitable for use with monaural audio signals of a limited frequency bandwidth, i.e., up to 20 KHz, has been less than completely satisfactory.
Television programming with stereo rather than monaural program audio has recently become available. Stereo audio signals, which include higher frequency components to carry the additional stereo information (L-R) and a second audio program (SAP), have an increased frequency bandwidth on the order of 100 KHz. A stereo audio FM modulator requires precise phase and frequency linear modulation of the audio carrier because any intermodulation between the signal components due to modulator non-linearity will result in noticeably degraded stereo reception. Accordingly, an FM modulator suitable for use with frequency modulated stereo audio signals must be extremely linear. The prior circuits for generating the modulated audio carrier in video signal distribution systems have been found wanting for quality stereo transmission.
The non-linearity of the audio carrier modulation results from the use of an inherently non-linear voltage controlled oscillator in generating the modulated audio carrier. Voltage controlled oscillators operating above 1 MHz are inherently non-linear due to the use of varactor technology. Indeed, voltage controlled oscillators operating at RF frequencies utilize "linearization" networks to improve linearity. However, a linearization network is also inherently non-linear. Thus, the output of the voltage controlled oscillator is insufficiently linear. Accordingly, prior headend systems using voltage controlled oscillators operating at RF frequencies do not have sufficient linearity for modulating a stereo audio carrier.
U.S. Pat. No. 4,546,387 (Glaab) issued on Oct. 8, 1985 and is assigned to the assignee herein. The Glaab patent illustrates circuitry for providing accurate spacing of video and audio carriers. While the circuitry disclosed in Glaab does provide very accurately spaced carriers, it is noted that its voltage controlled oscillators operate at above 60 MHz. Accordingly, such oscillators would be insufficiently linear to provide accurate frequency modulation of a stereo audio signal. The present invention utilizes voltage controlled oscillators operating below 1 MHz to provide accurately spaced and extremely linear FM modulated audio.
Accordingly, it is an object of this invention to provide an improveed FM modulator.
It is another object of this invention to provide an FM modulator having an extremely linear modulation characteristic.
It is another object of this invention to provide a video signal distribution system in which the audio carrier frequency is derived from the video carrier frequency.
It is a further object of this invention to provide audio FM modulation on a video signal particularly suitable for stereo audio transmission.
It is a further object of this invention to provide a television signal with stereo audio having accurately spaced video and audio carriers.